A source close to the investigation said Woerth will be charged with illegally raising funds for Sarkozy's 2007 campaign and "fraudulent abuse of a person's weakness" after he allegedly received cash from Bettencourt.

The case is only one of several corruption investigations plaguing the right-wing incumbent as he prepares for a tough re-election fight against Socialist flag-bearer Francois Hollande in a two-round vote in April and May.

Bettencourt is also at the centre of a series of long-standing, overlapping legal inquiries, including claims that she showered leading right-wing figures with envelopes stuffed with undeclared campaign donations.

Bettencourt's accountant, Claire Thibout, has testified to having provided 50,000 euros (ê65,000) in cash to Bettencourt's financial manager, Patrice de Maistre, which was then handed over to Woerth for Sarkozy's campaign.

She had allegedly been asked to provide 150,000 euros but did not have the cash at hand. Under France's electoral code, individual election campaign contributions may not exceed 4,600 euros.

A court has since found that 89-year-old Bettencourt, the billionaire heiress to the L'Oreal cosmetics and shampoo empire, is suffering from dementia and Alzheimer's disease and has placed her under guardianship.

Woerth left the government in 2010 and in 2011 police carried out searches of his home and the UMP's offices in connection with the case.

He has strongly denied the allegations.

"Nicolas Sarkozy's camp is completely clean," Woerth said in October, denouncing the allegations as a "shameless" smear campaign by "left-wing media".

Sarkozy vowed to lead an "irreproachable republic" when he came to office, but his camp has since been tainted by a series of scandals.

Another of the most high-profile cases has been the so-called Karachi affair, in which two close aides to Sarkozy have been charged by judges investigating alleged kickbacks on a Pakistani arms deal.

That case dates back to Sarkozy's time as budget minister, when he allegedly authorised the creation of a shell company used to channel the kickbacks to then-prime minister Edouard Balladur's unsuccessful 1995 presidential bid.

Magistrates are probing whether a 2002 Karachi bombing that killed 11 French engineers was revenge for the cancellation of bribes secretly promised to Pakistani officials.

Sarkozy and his camp have also been accused of ordering an illegal police investigation to identify an official leaking information on the Bettencourt scandal to a journalist from the newspaper Le Monde.

Judges have charged both a prosecutor close to Sarkozy and the head of France's DCRI domestic intelligence agency, Bernard Squarcini, with having illegally obtained the journalist's mobile phone logs in 2010.

Sarkozy, who as president is immune from criminal prosecution, has fiercely denied any personal wrongdoing.

But Hollande has taken a strong lead ahead of April's vote, with opinion polls showing him taking 28 to 30 percent of votes in the first round, ahead of 23 to 24 percent for Sarkozy, who has yet to officially declare his candidacy.

The pair could face a run-off on May 6 and Hollande is firm favourite.